The present invention relates generally to electronically commutated DC motors (i.e., brushless DC motors) and, more particularly, to a system and method to detect the presence or absence of a motor connection.
Brushless direct current (BLDC) motors are well known in the art. The phase windings in these motors are sequentially energized at appropriate times so as to produce a rotating magnetic field relative to a permanent magnet rotor. The timing of such energization is a function of where the permanent magnetic rotor is relative to a phase winding that is to be energized. Various means have been heretofore used to sense the position of the permanent magnet rotor relative to the phase windings. These have included optical sensors and Hall effect devices which feed a position signal to switching logic that selectively switches power on and off to the respective phase windings. However, such sensing devices add cost and complexity to a system, and may moreover require maintenance from time to time to assure continued proper operation. In certain high flux/power applications, such as those employing 350 volt motors, the Hall sensors are a common point of failure. As a result of these drawbacks, attention has recently been focused on “sensorless” systems which are not premised on any direct sensing of the rotor position itself. These systems generally attempt to measure the effect of the back electromotive forces produced in the energized windings by a rotating rotor. These systems have achieved various degrees of success in accurately measuring the effect of this back electromotive force.
Traditionally, detection that a motor is connected to drive electronics may be detected in one of two ways. First, sensors may be employed which provide feedback of motor position and motion thereby providing information about the motor being physically connected. However, as discussed above, reliance on such sensors complicate motor design and add cost.
Second, current may be driven through motor windings at a level that is sufficient for the drive electronics to measure. If voltage is increased high enough, and there is no current, a motor is not connected. This is feasible on sensorless systems; however, it takes hundreds of milliseconds to detect the presence of a motor. Additionally, ramping up motor current to a predefined level will almost always cause the motor to move, making starting more difficult. In some applications, it may not be desirable to move the motor by performing such a test. Moreover, current sensing may impose a requirement that custom parameters be used for each motor/drive situation. In cases where a custom parameter is not used, a high power drive could damage a small rotor (e.g., demagnetize).
Current motor drive technology simply attempts to restart a motor infinite times if no motor is plugged or operably connected. This approach is undesirable since it does not provide adequate fault isolation. Instead of being able to differentiate between an unplugged motor and a true start failure (e.g., due to external disturbance), the current approach simply posts start failures until the error stack fills with start failures.
Thus, it is desired to determine if a motor is connected to a solid state motor control assembly to isolate a potential fault without requiring a sensor and significant energy to be delivered to the motor itself.